This invention relates to variable reluctance actuators of either the linear or rotary type, and particularly to those whose mechanical force or position may be controlled throughout a range of movement of their movable element.
Variable reluctance electromagnetic actuators are well known in the art as exemplified by the linear motion solenoid devices shown in U.S. Pat. Nos. 3,671,814, 4,434,450 and 4,450,427. Although such devices disclose the possibility of controlling the force imposed by such actuators in a constant, controlled manner independent of actuator position, in practice they are unable to obtain this result. For example, in U.S. Pat. No. 3,671,814, a flux sensor is placed in the variable gap of the actuator's magnetic circuit for controlling coil current such that the magnetic field experienced by the flux sensor remains constant independent of position of the actuator. Although holding the field in the variable gap constant theoretically should produce constant force, in reality motion of the actuator changes the boundary conditions of the magnetic field such that the force produced varies significantly with motion. If the flux sensor is not placed in the variable gap, as in U.S. Pat. Nos. 4,434,450 and 4,450,427, a further variable is introduced because, as the actuator retracts, flux leakage circumventing the variable gap increases. Accordingly, holding constant the magnetic field experienced by such a fixed gap flux sensor likewise does not usually produce constant force independent of motion. Moreover, permitting the variable gap to close completely upon retraction, as taught by the latter two patents, further varies the actuating force by increasing it abruptly as the actuator nears full retraction.
None of the aforementioned variable reluctance actuators has a built-in capability for position sensing or position control between two stop positions. However, an integral means of position control for such variable reluctance actuators is disclosed in the copending, commonly-owned U.S. patent application of one of the inventors herein, Ser. No. 639,187, filed Aug. 9, 1984. As disclosed in such patent application, coil current which produces the actuator's magnetic field, and the instantaneous magnetic flux density of such field, are sensed concurrently and signals representative of each are fed to a divider which divides the coil current magnitude by the flux density magnitude, yielding a signal proportional to actuator position. Such a system, however, requires both a flux sensor and a divider in the position-sensing circuit which is costly. U.S. Pat. No. 3,413,457 discloses a general-purpose analog computer circuit using a Hall effect sensor as a divider in a constant-reluctance magnetic circuit. However, there is no suggestion of how such principle could be applied to a variable reluctance magnetic circuit to indicate position of a movable element.